60 FORMATION OF SHELLS OF ANIMALS, ETC., 
Hence we arrive at a most important physical fact, namely, 
that spherical bodies, formed on the principle of universal 
attraction in a medium of given density, become, in one 
of greater density, gradually disintegrated, and _ their 
molecules, at first completely separated, afterwards re- 
arrange themselves in fresh forms. 
The experiments next to be described, showing the facts 
of molecular disintegration, have been given already, in 
the ‘Transactions of the Microsopical Society,’ published 
in the January number of 1858 of the ‘ Quarterly Journal 
of Microscopical Science.’ These facts can be demonstrated 
in a description of artificial calculi presenting characters 
different im some respects from those already described. 
They may be prepared by dissolving one pound of gum 
arabic in two pints of water, and straining the mucilage 
through a fine hair-sieve, and then putting one pint of 
the solution, with two ounces of carbonate of potash well 
mixed together, into a quart bottle, after twenty-four 
hours adding, by means of a syphon, the other pint of 
mucilage, and after that leaving the bottle at rest for six 
weeks or two months, when the calculi will be found ad- 
herent to its sides, or in the fluid at the surface. Besides 
these calculi of different degrees of solubility, crystals 
are formed in these solutions; some consist of bicar- 
bonate of lime, formed by the free acetic acid in the 
mucilage combining with a portion of the potash, and 
setting free carbonic acid, which, uniting with some 
of the carbonate of lime, forms a bicarbonate. These 
crystals, being sparingly soluble, remain in the solutions, 
and chiefly on the surface. I may observe, that in a 
solution of gum all crystals produced by double decom- 
position, which do not combine chemically with it, are 
large and well-formed. Hence such a process may be 
